The Austin Community College electron transport page explains that as electrons travel down the transport chain, the electrons are transferred between protein complexes and lose energy that is used to pump protons across the mitochondrial membrane. This enables the maintenance of a proton gradient across the inner mitochondrial membrane, which in turn is used to run the ATP synthase to make the ATP that cells use for energy.
The electron transport chain is part of aerobic respiration. This enables much more efficient use of stored energy and food by the cell. In anaerobic respiration, the only energy that the cell can use is what it gets out of glycolysis. Glycolysis is the process by which cells break down glucose and only produces a net of 2 ATP molecules. Aerobic respiration, including the electron transport chain, produces a total of 36 ATP.
The protein complexes involved in the electron transport chain contain several proteins that are "electron carriers." These proteins are able to transfer electrons because each successive protein requires a lower energy level for the electron. In order to make the proton gradient, protons are pumped across the mitochondrial membrane at several points in the electron transport chain. However, that proton gradient is only used to make ATP by the ATP synthase, which is technically separate from the electron transport chain.